<p>A common feature of most fiber plants is their similar biological composition, an organic and mineral conglomerate mainly in the form of cellulose, lignin, hemicellulose, inorganic materials, and extractive substances such as pectin, fats, wax, and other. In particular, simple sugars formed in an aqueous environment inhibit the process of hydration of the cement binder. In paper extensive laboratory tests were conducted on the effect of different types and methods of organic filler mineralization on the compressive strength of the composite. They focused on eliminating the problem related to the instability of cement composites containing organic fillers due to the presence of simple sugars. Based on the test results the authors proposed a two-stage mineralization of organic filler in cement composites. First, an extractant was selected in the form of an aluminum sulfate solution, which leaches simple sugars from the organic aggregate (mainly from its surface layers) and partially neutralizes them, and then a neutralizer was selected in the form of a calcium oxide solution, whose task is to bind carbohydrates and increase the pH of the solution from acid to alkaline. The optimal contents of aluminium sulphate and calcium oxide in relation to organic fillers were also experimentally determined. The use of two-stage mineralization of organic fillers allowed an increase of the compressive strength of composites by approximately 70%, an increase of volume density as well as significantly reducing their water absorption. This solution enables more efficient use of organic waste in building materials, which is consistent with the idea of sustainable development.</p>